SAN FRANCISCO -- Pacific Gas and Electric Co. knew that the major natural-gas line that failed in a test near Bakersfield last week had ruptured nearly 40 years ago because of a flawed seam weld, but the company long relied on an improper inspection method for the pipeline that was not capable of finding such problems, The Chronicle has learned.

The longitudinal seam flaw found in 1974 during a test on the pipe that carries gas from the Arizona border to the Bay Area was the same defect that caused last year's deadly explosion on a transmission pipeline in San Bruno. However, as on the San Bruno line, PG&E persisted in vouching for the pipe's safety by using an inspection method that is suited mainly for finding corrosion, not bad welds.

A federal pipeline safety official said that under a pipeline safety law passed in 2002, PG&E should have been using tests designed to catch flawed welds on the Central Valley line because of the earlier rupture. Instead, the company didn't start conducting them until after the San Bruno disaster in September 2010.

It was such a test - in which the pipeline is shut down and filled with water boosted to high pressure levels - that found another flawed weld Oct. 24 on the line just west of Bakersfield.

The 1974 test tore an 18-inch gash along a longitudinal seam of the pipe, known as Line 300B, one of two parallel lines that supply the Bay Area with much of its natural gas. PG&E has replaced 100 feet of the pipe, intends to pressure test another 29 miles and hopes to put the line back in service in time for winter.

'Part of a pattern'

PG&E's past inspection practices on Line 300B are "part of a pattern of failing to comply with the regulations and the law of common sense," said Richard Kuprewicz, a pipeline safety expert and consultant to a ratepayers group, The Utility Reform Network. "It shows a serious lack of respect, a serious disregard for what these lines are capable of and of what can happen when gas transmission pipelines rupture."

PG&E officials have conceded that the company's pipeline inspection efforts before the San Bruno disaster were lacking, and say they have taken steps to improve them.

"We have openly acknowledged the need to significantly improve many of our historic integrity management practices," said Brian Swanson, a company spokesman. "Since the tragedy in San Bruno, we have been working very intensively to do exactly that.

"We are taking whatever steps necessary to bring our pipeline testing and inspection programs up to industry-leading levels and to assure our customers that our system is operating safely," Swanson said.

Bad weld near Coalinga

The earlier rupture on Line 300B happened during a pressure test on a section of pipe near the Harris Ranch in Coalinga (Fresno County), about 100 miles north of where the line failed last week. A PG&E in-house analysis of the June 20, 1974, incident described the failure as being caused by a "poor weld."

"It is evident that the cause of the failure was a defective longitudinal seam weld," said the document, which PG&E provided to state regulators investigating how it accounted for weld problems in its system. "This weld has poor penetration (less than 50 percent in some places) over a length of approximately 18 inches."

In subsequent years, PG&E inspected Line 300B mainly with a test called direct assessment. That method is cheaper than pressure tests and does not require pipeline shutdowns, but cannot find bad welds. Instead, the test is best suited for finding pipeline corrosion.

Until the San Bruno blast, PG&E conducted most of its transmission-line inspections with direct assessment. It has since announced plans to use pressure tests for hundreds of miles of pipeline over the next few years.

Under federal law, the company should have been barred from using direct assessment on Line 300B once it had evidence of a previous seam-weld rupture anywhere along the pipe's more than 500-mile route, said an official with the government's pipeline safety agency.

Wrong method under law

"Direct assessment cannot be used to assess manufacturing- and construction-related defects," said Damon Hill, a spokesman for the U.S. Pipeline and Hazardous Materials Safety Administration. "Seam failure would be a manufacturing and construction defect - so the proper assessment" would be either high-pressure water tests or automated in-line devices known as smart pigs.

"You can only use tools authorized to check for seam failure," Hill said.

Besides the 1974 failure, Line 300B and its parallel line, 300A, both have a history of unexplained leaks.

PG&E logged five leaks of unknown cause along a 20-mile stretch of Line 300B south of Bakersfield in the 1950s, company documents show.

Line 300A also had two unexplained leaks, one in 1951 and one in 1999, as well as several troubling incidents recently.

In September 2009, the pipe failed at a longitudinal seam weld near Needles (San Bernardino County), a problem discovered during a routine leak inspection. PG&E documents show an earlier defective seam weld caused a leak in 2002 about 183 miles northwest of Needles.

No mention to state

Neither those problems nor the 1974 rupture were mentioned on pipeline data reports that PG&E provided to the state to justify its use of corrosion-only inspections. Experts called the omission inexcusable.

"You have to look at the history and all types of failures - if you don't, you're not identifying all the threats under law," said Royce Don Deaver, formerly an engineering adviser for Exxon and now a pipeline safety consultant. "If you have a failure, you are obligated to investigate the cause and you are supposed to identify corrective measures for the whole system."

To Deaver, PG&E's treatment of Lines 300A and B were reminiscent of how it handled the pipeline that exploded in San Bruno, a disaster that killed eight people and destroyed 38 homes. In that case, the company knew of at least 26 unexplained leaks dating back decades, any of which could have been caused by a bad weld, yet never inspected for such a problem.

"Basically, they do what they think they can get away with," Deaver said. "The law is only relevant if they get caught."

PG&E's Swanson said the company's focus is on "taking immediate and aggressive action" to assure the safety of the company's pipelines. He noted that it was PG&E's decision to pressure test Line 300B that resulted in finding the flaw in the line last week.

"Stories about what we could have done better in the past have already been shared numerous times," Swanson said. "Our customers need to know that we've learned valuable lessons from the past and that we're implementing what we learned toward building one of the safest gas systems in the country."

Complete coverage: To see Chronicle reports on pipeline safety since the 2010 explosion in San Bruno, video of the disaster and government investigative documents, go to www.sfgate.com/san-bruno-fire.